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I'll explain everything now =)
Level 1:
Imp - with 2-3 damage combined with Battle Fury, it's already quite strong 8)
Level 2:
Fury - no need to comment. Highest damage, initiative, and speed, excellent abilities.
Level 3:
It's more complicated here =) I hate these guys, but still, I choose Ghosts. Their ethereal nature significantly affects the tactics, and that's better than any damage or HP differences.
Level 4:
Even more complicated: they're all strong. Let's eliminate those who are just strong (Druids, Vampires, and Mages). The ones left have extremely interesting abilities. Ultimately, Griffons, especially the Royal ones, have a very diverse range of tactical options.
Level 5:
We have opposing forces: Nightmares and Unicorns, Inquisitors, Liches, Hydras, and Djinn Sultans. Here, we can highlight the super-awesome mega-cannon fodder of all times and peoples - Cave Hydras. You can safely send them into battle, especially it's fun to teleport them into the thick of enemies who haven't retreated from their positions. An unreal amount of HP + regeneration + an unyielding area attack overshadows even the abilities of horses, both good and evil, each of which, if successful, will take out some unit.
Level 6:
Probably the most difficult choice 8)) Let's eliminate the mundane Paladins, Twilight Witches, and Rakshasa Rajahs. Three completely unique creatures remain. Death Knights are still slightly behind the remaining two.
So, we have: Pit Lord (the highest damage among all level 6 troops + kills any enemy + and also throws spells) and Ent. My choice: Ent. He stands out too much. So, Ent has the highest amount of HP among not only level 6, but also half of level 7 units; rooting, a response to all attacks + 50% defense - teleport him into the thick of the enemy and let him stay there. Plus, he can hold back powerful enemy units like dragons with roots, and dragons will either have to wait or attack the Ents, which is usually not desirable if there are Druids, Archers, etc. Again, the choice fell on the tactical component of the unit.
Level 7 - Black Dragon. No need to comment here - it's very obvious: the strongest unit.
Elementals: I'll give preference to Water Elementals, it seems quite logical to me because they can cast spells twice)